Earth appears to have a natural climate control system that has helped keep the planet habitable for more than 100 million years. Scientists have long known that this system exists, but the mechanisms behind it have remained difficult to explain.
New research points to a previously overlooked connection between sea level and the availability of phosphate in the ocean. Changes in global temperature affected the size of polar ice sheets, which altered sea level. Those shifts then influenced how much phosphate reached the open ocean, how much carbon became buried in marine sediments, and how much carbon dioxide remained in the atmosphere.
Together, these processes helped determine whether Earth became warmer or cooler over long stretches of time.
The new study was co-authored by Zunli Lu, professor of Earth and environmental sciences in Syracuse University's College of Arts and Sciences. It examines how changing sea levels and ocean oxygen conditions affected phosphate availability and atmospheric carbon dioxide over the past 60 million years.
The findings were published in Proceedings of the National Academy of Sciences .
"We know that atmospheric carbon dioxide decreased substantially as Earth cooled over the last 60 million years, but we have had remarkably little understanding of where that carbon ended up," says lead author Ros Rickaby, professor of Earth sciences at the University of Oxford, in a department news article. "Our results suggest that enhanced burial of organic carbon in marine sediments played a much more important role than was previously appreciated."
At the center of the study is phosphorus, especially phosphate, a nutrient that marine organisms need to grow. The researchers describe phosphate as a previously "invisible" part of the climate puzzle.
When sea levels were high, shallow continental shelves covered a larger area. These shelves trapped phosphate in coastal sediments, leaving less of the nutrient available in the open ocean.
With less phosphate in the water, marine productivity slowed. Fewer organisms grew, less organic carbon sank to the seafloor, and less carbon became buried in sediments. Ocean waters also became more oxygen-rich, while carbon dioxide accumulated in the atmosphere.
Falling Seas Triggered a Carbon Feedback
When sea levels dropped, the process moved in the opposite direction.
As continental shelves shrank, more phosphate entered the water. That extra nutrient supported a surge in marine life. When organisms died, their remains sank and decomposed, consuming oxygen in the surrounding water.
Over time, low-oxygen zones formed in the ocean. When those zones reached carbon-rich sediments on continental shelves, they activated a powerful feedback process.
Low oxygen caused sediments to release even more phosphate. That additional phosphate encouraged more marine growth, which led to greater burial of organic carbon on the seafloor. As more carbon was removed from the ocean and atmosphere, atmospheric CO2 declined.
"Our co-author, Christian Bjerrum, studied the connection among sea level, ocean oxygen and phosphate with a computer model two decades ago," Lu says. "We finally pieced together the geologic records necessary to test this hypothesis."
A Sea Level Sweet Spot for Carbon Burial
The researchers found that this feedback reached its greatest strength when sea level stood roughly 10 to 40 meters above its modern level.
At this sea level "sweet spot," low-oxygen waters overla…